1. Field of the Invention
The present invention relates to an ink jet head that ejects ink as ink droplets, and a recording apparatus and a recording method using the ink jet head. More specifically, the present invention relates to an electrostatic ink jet head that controls ejection of ink containing charged fine particles by means of an electrostatic force, an ink jet head where energy required for ink ejection is reduced, and to a recording apparatus and a recording method with which an image is recorded on a recording medium using the ink jet head.
2. Description of the Related Art
An ink jet recording apparatus records an image on a recording medium by ejecting ink containing a colorant from ejection ports as ink droplets, which then fly and impinge on the recording medium. According to an ink droplet ejection method, there are known the ink jet recording apparatuses of an electrostatic system, a bubble system, a thermal system, a piezoelectric system, and the like.
The electrostatic ink jet recording system is a system in which ink containing a charged fine particle component is used and an image corresponding to image data is recorded on a recording medium by controlling ejection of the ink by means of an electrostatic force through application of a predetermined voltage to each ejection electrode of an ink jet head in accordance with the image data. As a recording apparatus adopting this electrostatic ink jet recording system, there are known ink jet recording apparatuses disclosed in JP 10-138493 A, JP 11-078026 A, and JP 09-254372 A, for instance.
FIG. 22 is a conceptual diagram schematically showing an example of an outlined construction of an ink jet head of the ink jet recording apparatus disclosed in JP 10-138493 A described above. An ink jet head 200 shown in FIG. 22 includes a head substrate 202, an ink guide 204, an insulating substrate 206, an ejection electrode 208, a counter electrode 210 supporting a recording medium P, a bias voltage source 212, and a signal voltage source 214. Note that in this drawing, only one individual electrode serving as an ejection means constituting the ink jet head disclosed in JP 10-138493 A is conceptually illustrated.
Here, the ink guide 204 is made of a resin flat plate having a predetermined thickness and including a protrusion-like tip end portion 204a, and is arranged on the head substrate 202. Also, in the insulating substrate 206, a through hole 216 is formed at a position corresponding to a position at which the ink guide 204 is arranged. The ink guide 204 passes through the through hole 216 formed in the insulating substrate 206 and its tip end portion 204a protrudes upward from the upper surface of the insulating substrate 206 in the drawing, that is, from a surface thereof on a recording medium P side. Also, the head substrate 202 and the insulating substrate 206 are arranged so as to be spaced apart from each other by a predetermined distance, and a flow path 218 of ink Q is formed between these substrates 202 and 206.
Also, the ejection electrode 208 is provided in a ring shape for each individual electrode on the upper surface of the insulating substrate 206 in the drawing so as to surround the through hole 216 formed in the insulating substrate 206. The ejection electrode 208 is connected to the signal voltage source 214 that generates a pulse signal corresponding to ejection data (ejection signal) such as image data or print data, and the signal voltage source 214 is grounded through the bias voltage source 212.
In addition, the counter electrode 210 is arranged at a position opposing the tip end portion 204a of the ink guide 204 and is grounded. Also, the recording medium P is arranged on the lower surface of the counter electrode 210 in the drawing, that is, on a surface thereof on an ink guide 204 side, and the counter electrode 210 functions as a platen of the recording medium P.
In the ink jet head 200 constructed in this manner, at the time of recording, ink containing a fine particle component charged to the same polarity as a voltage applied to the ejection electrode 208 is circulated by a not-shown ink circulation mechanism in a predetermined direction (from the right to the left, in the illustrated example) in the ink flow path 218, and a part of the ink Q in the ink flow path 218 is supplied to the tip end portion 204a of the ink guide 204 through the through hole 216 in the insulating substrate 206 by capillary action or the like.
Here, a predetermined high voltage (DC voltage of 1.5 kV, for instance) is constantly applied to the ejection electrode 208 by the bias voltage source 212. Under this state, the strength of an electric field in proximity to the tip end portion 204a of the ink guide 204 is low and the ink Q supplied to the tip end portion 204a of the ink guide 204 will not fly out from the tip end portion 204a. In this case, however, a part of the ink Q in the ink flow path 218, in particular, the charged fine particle component moves upward above the upper surface of the insulating substrate 206 in the drawing while passing through the through hole 216 in the insulating substrate 206 and aggregate in the tip end portion 204a of the ink guide 204.
On the other hand, when a pulse voltage, e.g., DC voltage of 500 V (ON-time; 0 V: OFF-time) is applied by the signal voltage source 214 to the ejection electrode 208 biased to the high voltage (DC 1.5 kV) by the bias voltage source 212, both of these high voltages are superimposed on each other and 2 kV is applied to the ejection electrode 208, for instance. As a result, the ink Q, in particular, the charged fine particle component in the ink Q further moves upward along the ink guide 204 and aggregate in the tip end portion 204a. Then, the ink Q aggregating in the tip end portion 204a of the ink guide 204 and containing the charged fine particle component flies out from the tip end portion 204a by means of an electrostatic force, is attracted by the grounded counter electrode 210, and adheres on the recording medium P. In this manner, a dot is formed by the charged fine particle component.
By forming dots of the charged fine particle component in this manner while relatively moving the ink jet head 200 and the recording medium P supported on the counter electrode 210, an image corresponding to image data is recorded on the recording medium P.
With this ink jet recording apparatus, the ink Q is guided by the ink guide 204 provided in an ejection port and an ink droplet R flies out from the tip end portion 204a, so that it becomes possible to stabilize the flying of the ink droplet R.
Meanwhile, in JP 11-078026 A, an image forming apparatus is disclosed which uses a head obtained by providing a control electrode below the ink flow path 218 in the ink jet head 200 described above. During recording, this control electrode causes the ink Q in the ink flow path 218, in particular, the charged fine particle component in the ink Q to migrate toward the ejection electrode 208 and further toward the tip end portion 204a of the ink guide 204. On the other hand, during non-recording, the control electrode causes the ink Q adhering to the ink guide 204 and the charged fine particle component in the ink Q to migrate toward a lower portion of the ink flow path 218.
Also, in JP 09-254372 A, an ink jet head is disclosed in which parallel ejection electrodes (parallel electrodes) provided in a groove-like ink flow path are used in place of the ring-like ejection electrode (circular electrode) disclosed in JP 10-138493 A and JP 11-078026 A.
Further, in JP 2002-273893 A, an ink jet printer nozzle is disclosed in which a pin serving as an ink guide is provided inside an ejection port (nozzle) and, when an ink droplet flies out from the ejection port, a tail portion of the ink droplet is cut by an end portion (protrusion) of the pin. With this construction, an ink meniscus in the ejection port is stabilized at the time of flying of the ink droplet and cutting of the ink droplet.
Also, in JP 01-222970 A, a liquid injection recording head is disclosed in which a rod-like guide is provided inside an ejection port and is subjected to treatment imparting hydrophilic property thereto. With this construction, at the time of ejection of an ink (recording liquid) droplet, the ink droplet is ejected along the rod-like guide, which stabilizes an ejection direction of the ink droplet.
By the way, even when the ink jet heads disclosed in JP 10-138493 A, JP 11-078026 A and JP 09-254372 A described above are used, in the case of a recording apparatus that is required to perform high-definition recording at a high speed, a line head is necessary, which is capable of recording images of one line at a time inevitably. When the definition and recording speed of the recording apparatus are respectively 1200 dpi (dot/inch) and 60 ppm (page/minute), for instance, a line head that is capable of recording an image on a recording medium having a width of 10 inches needs to include as numerous as 12000 individual electrodes, whose number is equal to the number of pixels on one line, and pulse voltage sources, that is, drive circuits whose number is equal to the number of the individual electrodes to be driven.
In this case, in the line head, the individual electrodes and the pulse voltage sources need to be implemented at a physically extremely high density with respect to the line direction. The pulse voltage sources use a high voltage (around 400 to 600 V, for instance), so that when the individual electrodes and the pulse voltage sources are arranged at a high density, there involves a high risk of causing the discharge. Accordingly, it is extremely difficult to cope with both high-density implementation and high-voltage driving. Note that in order to apply pulse voltages to the ejection electrodes, the pulse voltage sources are required to generate the pulse voltages. Here, the ejection electrodes are each a small electrode, so that the amount of a current consumed by ejection itself is small. However, if high pulse voltages are generated by the pulse voltage sources, current consumption is increased. Also, the pulse voltage sources consume currents in order to generate the pulse voltages, so that if high pulse voltages are generated, the current consumption is increased. When the number of individual electrodes is small, the increased current consumption causes little problem. However, when a large number of individual electrodes are used as described above, the increased current consumption causes a problem.
In addition, in the case of the apparatuses disclosed in JP 10-138493 A, JP 2002-273893 A and JP 1-222970 A described above, the ink is pulled out or pushed out through a gap with the ink guide provided in a tight space of the ejection port, so that a large force is required in order to cause ink ejection. That is, it is required to increase the pulse voltages to the ejection electrodes in the case of the electrostatic system, to increase electric power to heating elements in the case of the bubble system or the thermal system, and to increase electric power to piezoelectric elements in the case of the piezoelectric system. Consequently, there arises a problem in that loads on an electric circuit and the like are increased and operation stability is lowered.
Also, at the time of activation of the apparatuses (at the time of start of recording), a long time is taken to supply the ink to the ejection port, so that a delay time from exertion of an ejection force to actual ejection is elongated, during which it is impossible to eject an ink droplet having a predetermined size set for the ejection force. Consequently, there involves a problem in that the dot sizes of first several dots become small and a print failure occurs.
Further, in JP 1-222970 A, the ink guide is made ink-receptive in order to enhance a contact property of the ink to the ink guide, which causes a problem in that the larger ejection force is necessary for ink ejection.